The invention relates to plating methods and plated products. In particular, the invention relates to a multi-layer plating method on a metal surface, and especially on a metal film on a plastic substrate, and most especially to a plastic substrate of ABS (acrylonitrile butadiene styrene) with an electroless metal coating.
As is well known in the art, such plating occurs by making the metal, or any conductive substance, the cathode in an electrolytic cell in which current is applied at the desired current density.
Plated plastic articles are known having an electroless metallic film such as copper on the plastic, an electrolytically deposited copper (usually bright copper from the cyanide or sulfate), bright nickel (from Watts nickel or the sulfate), followed by chromium, including microcracked chromium. Copper and nickel fluoborates, with suitable brighteners, have been used in electroplating baths in such a process. U.S. Pat. No. 3,748,236 discloses a brightener for such a system. Fluoborate plating systems are known to provide advantages in plating speed, ease of bath preparation from liquid concentrates, reduced pollution and easy process control.
Traditional copper-nickel-chromium systems have provided a satisfactory level of brightness, ranging from semi-bright to full bright. However, plated articles, and particularly plated plastic articles, with a copper sublayer often exhibit unsightly (green) corrosion products. Such plated plastic articles may not withstand temperature cycling including temperatures below 0.degree. C.
Multi-layer plating systems on conductive substrates such as steel have been known including a wide variety of metals or alloys for each layer. For example, U.S. Pat. No. 2,428,318 discloses coated steel articles with at least three layers of at least two different metals, said to provide increased rust resistance with thinner layers. One product is coated with 0.0005 inch nickel, 0.001 inch lead and 0.0005 inch nickel.
U.S. Pat. No. 2,658,266 discloses a metal substrate coated with a nickel flash (from the sulfate and chloride), lead (from the fluoborate), bright (preferably buffed) antimony, a nickel flash and chromium. The stated purpose is to minimize the use of costly nickel and to form a brighter finish and greater corrosion resistance than with certain prior art multi-layers. This patent states that the lead layer may be omitted if the steel is first roughened. The antimony layer is not stated to be dispensable. In accordance with the stated purpose of conserving nickel, the thickness of the first nickel flash is preferably 0.01 to 0.1 mil (although up to 0.5 mil) and of the second flash 0.001 to 0.05 mil (preferably 0.002 to 0.03).
U.S. Pat. No. 2,714,088 discloses a steel article coated with lead, antimony, nickel and chromium.
U.S. Pat. No. 3,009,238 discloses a composite coating on steel including a plurality of electro deposited nickel layers and a layer of a dissimilar metal, including cobalt, tin or lead, between each nickel layer.